Optical communications networks, and more particularly passive optical communications networks, are increasingly used to give network access to residential or office gateways, or data centres, or to ensure mobile backhauling for instance.
In an attempt to increase the number of user terminals to be served by one access system to the network, wavelength (or frequency) division multiplexing technologies have been developed. These technologies take advantage of multiplexing several optical signals using different carrier frequencies on a single optical fiber. Even though some user terminals may share the same carrier frequency, a frequency splitter is typically used to separate different frequencies in use, in order to increase the number of simultaneous optical transmissions. The frequency splitter is typically placed between the user terminals and a set of master terminals providing access to the rest of the network. For instance these master terminals provide access to a metropolitan network or a core network. Different techniques can be used to achieve such frequency splitting. We can cite thin films based systems, interference cavities as AWG (Array Wavelength Gratings) and FBG (Fiber Bragg Gratings) based systems.
The frequency splitter then comprises several optical band-pass filters, for each direction of communication. It is used to filter and combine optical signals issued by the user terminals toward the master terminal to which said user terminals are attached, each user terminal being then supposed to communicate with one master terminal. In the other direction, it is used to filter and spectrally split the optical signals issued by the master terminal toward the user terminals that are attached thereto.